Recently, safety in collisions has become an important issue regarding automobiles, and technology has been developed for the body of the automobile so as to ensure that a space for occupants will remain to ensure the survival and protection of the occupants in the impacts in automobile collisions. In actual collisions of automobiles, collisions may occur from the front, the sides, the rear, etc. Specifically, the collisions from the sides are important from the standpoint of the protection of the occupants. This is because a structural part of an automobile body, in this case, a center pillar is close to the car occupants.
Accordingly, a steel sheet in which strength, specifically, yield point thereof, is increased is used for structural parts of an automobile body, such as a center pillar, which may be important when a side collision occurs. The yield point of a material is an important characteristics of parts that may deform in side collisions because the parts should be prevented from deforming as much as possible when a collision occurs.
In general, when a steel sheet is strengthened, ductility thereof is decreased, and press formability is thereby decreased. Accordingly, the cross sectional shape of a part should be simple, and a steel sheet having relatively low strength should be used instead of the above steel sheet having very low press formability.
Currently, various steel sheets prescribed in JFSA2001 “Cold rolled steel sheets and strip for automobile use”, standardized by The Japan Iron and Steel Federation, are widely used for automobile bodies. Specifically, steel sheets having a grade of tensile strength of 590 MPa or 780 MPa are widely used for parts, such as a center pillar, which may receive the impact in side collisions. Therefore, if a steel sheet having higher strength can be used, the deformation of the automobile body that may occur in a collision is reduced. In addition, the steel sheet can be reduced in weight by decreasing sheet thickness thereof. However, this method is not easily performed. A steel sheet having a grade of tensile strength of 780 MPa as described above, that is, a steel sheet having a sheet thickness of 1 mm and prescribed by JSC780Y in JFSA2001, the total elongation is 14 to 27%, and the average value of the total elongation is approximately 21%. Therefore, if a steel sheet does not have an elongation of approximately 20% or more, the steel sheet cannot be substituted for a part that is made of a material prescribed by JSC780Y in the present circumstances.
A steel prescribed by JSC1180Y is a type of a steel having the highest strength of steel prescribed by JFSA2001. In a case of the steel prescribed by JSC1180Y and having a sheet thickness of 1 mm, the yield point is 825 to 1215 MPa but the total elongation is only 6 to 17%. In practice, the inventors performed tensile tests by using a test specimen of type of No. 5 that was prescribed by JIS Z 2201 and was made of a steel with grade of JSC1180Y, and the total elongation was approximately 8%. The steel having total elongation of such a degree cannot be used as a substitute steel sheet for a steel with grade of JSC780Y.
In the present techniques for steel sheet for automobiles, a steel having both high strength and high ductility is not easily produced because metal structure is strengthened by quenching so as to improve the strength of a steel. Since it is difficult to improve the yield point to be not less than 1000 MPa when a steel has a ferrite, a steel sheet may be quenched so as to have a structure mainly consisting of martensite. In this case, elongation of the steel sheet is deteriorated because the martensite has high strength but has low ductility.
A steel sheet having both high strength and high formability has been required, but such a steel may not be formed as long as a steel is mainly consisting of ferrite as described above. A typical austenite stainless steel consisting of austenite has relatively high strength and has a superior elongation compared to a steel consisting of ferrite. In this case, austenite stainless steel requires a large quantity of Ni and Cr, and alloy cost is thereby increased. Recently, a technique for providing both high strength and high ductility to a steel has been the subject of research. In this technique, in order to reduce the alloy cost, a steel having austenite is formed by reducing Ni and adding a large amount of Mn instead.
For example, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-507251 (WO99/001585) discloses a steel sheet having characteristics of TWIP (twining-induced plasticity) and TRIP (transformation-induced plasticity). This steel sheet is made of austenite steel including approximately 25% of Mn by weight and not more than 12% of the total of Si and Al by weight. This steel sheet has a yield point of not less than 400 MPa, a tensile strength of 1100 MPa, a uniform elongation of 70%, and a maximum elongation of 90%. Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-504175 (WO03/029504) discloses a duplex steel or a triplex steel including 0.5 to 2% of C by weight, 18 to 35% of Mn by weight, and more than 12% of the total of Al and Si. Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-509912 discloses a production method for a steel, in which a steel including 7 to 30% of Mn and 3.5 to 12% of the total of Al and Si is used, and yield strength of the steel is improved by forming at 2 to 25% at room temperature.
Every raw material of a steel sheet described in the above conventional techniques has austenite that is produced by adding a large amount of Mn thereto, and ductility is thereby improved. However, the above techniques have the following problems.
The steel sheet disclosed in the Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-507251 (WO99/001585) has high elongation and has a yield point of not less than 400 MPa. Although parts for receiving an impact of side collisions, such as a center pillar as described above, require a high yield point, the yield point of this steel sheet is insufficient. This steel sheet has a tensile strength of 1100 MPa, but this degree of tensile strength is insufficient because a much higher tensile strength is required so as to ensure a space for occupants in an automobile body.
In the Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-504175 (WO03/029504), a steel does not have a metal structure of a single phase of austenite, but has a mixed structure of ferrite and austenite, or a mixed structure of ferrite, austenite, and martensite. This steel sheet has a characteristic in which flow stress is more than 400 MPa, and a cold strip of the steel has strength of 900 MPa and maximum elongation of 70%. The strength of this steel sheet is insufficient for the required strength, which is the same as the case of the Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-507251 (WO99/001585).
The technique disclosed in the Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-509912 is a production method for improving yield strength and tensile strength by forming an austenite steel sheet including a large amount of Mn at room temperature. As a practical example, Table 1 in the Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-509912 shows material characteristics of a steel that includes 25.9% of Mn and was cold rolled at different rolling reductions. When the rolling reduction was 50%, flow stress at 0.2% was increased to 1051 MPa, but elongation was substantially decreased to approximately 5%, and this steel is therefore inappropriate for press forming. In the Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-509912, the rolling reduction should be decreased so as to ensure the elongation of not less than 20%, and the flow stress at 0.2% is thereby less than 1000 MPa.